recursive-barrel/generate.js

const fs = require('fs').promises
const path = require('path')

async function createBarrel(key, dir, n = 10, depth = 1) {
  if (depth === 0) {
    await fs.writeFile(
      path.join(dir, 'index.mjs'),
      `export const ${key} = ${JSON.stringify(key)}`
    )
    return
  }

  await fs.writeFile(
    path.join(dir, 'index.mjs'),
    Array(n)
      .fill(0)
      .map((_, i) => `export * from './${i}'`)
      .join('\n')
  )

  await Promise.all(
    Array(n)
      .fill(0)
      .map(async (_, i) => {
        await fs.mkdir(path.join(dir, i.toString()))
        await createBarrel(
          key + '_' + i,
          path.join(dir, i.toString()),
          n,
          depth - 1
        )
      })
  )
}

// 10^4 = 10,000
fs.mkdir(path.join(__dirname, 'dist')).then(() =>
  createBarrel('b', path.join(__dirname, 'dist'), 10, 4)
)